use of Documenter internals

[ericphanson]

e.g. most of https://github.com/ericphanson/Asciicast.jl/blob/6e708110fadf4b5c5b92a12142cac7fcaeed4809/src/runner.jl

maybe this should get upstreamed? or maybe there should be a committed API for this?

ref https://github.com/JuliaDocs/Documenter.jl/issues/1675#issuecomment-899125024